Uranium is produced from uranium-bearing ores by various procedures which employ a carbonate or acid lixiviant to leach the uranium from its accompanying gangue material. The acid lixiviants usually are formulated with sulfuric acid which solubilizes uranium as complex uranyl sulfate anions. The sulfuric acid normally is used in a concentration to maintain a pH between about 0.5 to 2.0. However mild acidic solutions such as carbonic acid solutions, having a pH between about 5.0 and 7.0 may also be employed. Carbonate lixiviants contain carbonate lixiviants may be formulated by the addition of alkali metal carbonates and/or bicarbonates or by the addition of carbon dioxide either alone or with an alkaline agent such as ammonia or sodium hydroxide in order to control the pH. The pH of the carbonate lixiviants may range from about 5 to 11. The carbonate lixiviants may also contain a sulfate leaching agent. The lixiviant also contains a suitable oxidizing agent such as oxygen or hydrogen peroxide.
The leaching operation may be carried out in conjunction with surface milling operations wherein uranium ore obtained by mining is crushed and blended prior to leaching, heap leaching of ore piles at the surface of the earth, or in situ leaching wherein the lixiviant is introduced into a subterranean ore deposit and then withdrawn to the surface. Regardless of the leaching operation employed, the pregnant lixiviant is then treated in order to recover the uranium therefrom. One conventional uranium recovery process involves passing the pregnant lixiviant through an anionic exchange resin and the elution of the resin with a suitable eluant to desorb the uranium from the resin. The resulting concentrated eluate is then treated to recover the uranium values, for example, by precipitating uranium therefrom to produce the familiar "yellowcake".
The anionic ion exchange resins employed for uranium concentration are characterized by fixed cationic adsorption sites in which the mobile anion, typically chloride or another halide, hydroxide, carbonate or bicarbonate, is exchanged by the uranyl complex anion. Such anionic ion exchange resins are disclosed, for example, in Merritt, R. C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines Research Institute, 1971, pages 138-147, which are hereby incorporated by reference. Suitable anionic exchange resins may take the form of polymers or copolymers of styrene substituted with quaternary ammonium groups or polymers or copolymers of pyridine which are quaternized to form pyridinium groups.
Several methods have been described to increase the efficiency of the ion-exchange resin either by extending the resin life by regeneration or by utilizing means to protect the resin from degradation. For example, U.S. Pat. No. 4,397,819 to Yan et al. relates to a process for restoring and maintaining the total ion-exchange capacity of the resin used for uranium recovery by treating the resin with a solution containing Na.sub.2 CO.sub.3 or NaHCO.sub.3, or admixtures thereof.
U.S. Pat. No. 4,298,578 to Yan et al. relates to a method for recovering uranium and/or related values which include means for protecting ion-exchange resins in the recovery operation from oxidative degradation due to contact with hydrogen peroxide. A guard chamber is positioned in the elution circuit so that barren eluant, after it is stripped of its uranium and/or related values by treatment with hydrogen peroxide, will flow through the chamber. The guard chamber contains catalytic material, e.g. activated carbon, which decomposes hydrogen peroxide upon contact into water and oxygen. The barren eluant, after it passes through the catalytic material, is used to make up fresh eluant for reuse in the recovery method without the risk of the fresh eluant causing oxidative degradation of the resins.
U.S. Pat. No. 4,235,850 to Otto, Jr. relates to a process for the recovery of uranium from a saline alkaline lixiviant employed in a uranium leaching operation. An ion exchange resin is employed to absorb uranium from the lixiviant. Prior to contacting the resin with the lixiviant, the pH of the lixiviant is reduced to a value of less than 7. By this technique, the resin loading in the presence of chloride ion is materially increased. The pH values for optimum resin laoding capacity decrease as the salinity of the lixiviant increases. Resin loading is enhanced by the presence of bicarbonate ion in the lixiviant.
As can be seen from the above, it is desirable to increase the efficiency of an ion-exchange resin system by either extending the life of the resin or by increasing the loading capacity of the resin.